Subsolidus Conditions Research Articles

Fabric Analysis in Upper Crustal Post-Collisional Granitoids from the Serre Batholith (Southern Italy): Results from Microstructural and AMS Investigations

The Serre Batholith in Central Calabria (southern Italy) represents the intermediate portion of a continuous cross-section of late Variscan continental crust. The various granitoid units of the batholith were emplaced at depths between 23 and 6 km through an overaccretion mechanism that, at its upper levels, was marked by the emplacement of two-mica granodiorites and granites (MBG) at c. 295 Ma, followed by weakly peraluminous granodiorites (BAG) at c. 292 Ma. These upper crustal granitoid rocks have recorded tectonic stresses, which affected the batholith during cooling of the magmatic bodies, exhibiting a range of deformation microstructures from submagmatic to low-temperature subsolidus conditions, but without developing an evident meso/micro-structural fabric. Anisotropy of magnetic susceptibility (AMS) was employed to identify a possible “internal” fabric of the Serre upper crustal granitoids, revealing a magnetic foliation represented by a mainly oblate AMS ellipsoid. Magnetic foliations and lineations are consistent with a stress field characterized by a shortening axis roughly oriented NW–SE. Further studies are in progress to investigate more in depth the relationships between regional tectonic structures and the emplacement of the late-Variscan Serre Batholith granitoids.

An Almahata Sitta EL3 fragment: implications for the complex thermal history of enstatite chondrites

Almahata Sitta is a polymict breccia, consisting of many kinds of clasts. Here we present our mineralogical and petrological results on an EL3 fragment, MS-177 from Almahata Sitta. This fragment shows a typical type 3 chondritic texture, consisting of well-defined chondrules, isolated silicate minerals, and opaque nodules. Most chondrules are enstatite-rich with some having olivine. Although these components are typical of EL3 chondrites, the mineral abundances and compositions are different from the other EL3s. Diopside is unusually abundant in MS-177. On the other hand, perryite and daubreelite were not found. The major pyroxene is orthoenstatite, and the silica phase is quartz. Fe–Ni metal has relatively high P contents. Troilite is enriched in Cr and Mn. Keilite and buseckite are present in MS-177. From the mineralogy and texture, MS-177 experienced a high-temperature event under subsolidus conditions. Shock-induced heating for a short duration might explain this high-temperature event. We suggest that other E3 chondrites also experienced heating events under such subsolidus conditions on their parent bodies. On the other hand, the high abundance of diopside cannot be explained by a secondary thermal event and may have been a primary feature of MS-177, formed before accretion to the parent body.

Multiple thermal events recorded in IIE silicate inclusions: Evidence from in situ U–Pb dating of phosphates in Weekeroo Station

Silicate-bearing iron meteorites record metal–silicate separation and mixing processes of planetesimals. Of particular interest is the IIE group as its silicate inclusions show great petrological diversity, probably resulting from the complex petrogenesis involving early differentiation and impact-induced dynamic mixing, re-melting, and reduction. These geologic records may have been documented by individual minerals to various extents, which blurs the chronological records on the formation of IIEs. Here, by combining X-ray elemental mapping with ion microprobe dating technique, we show that two groups of phosphates are present in the Weekeroo Station IIE iron meteorite. They have distinct petrography, mineralogy, Th/U ratios, and Pb isotopic compositions, which reflects their different formation mechanisms. One group (Group 1) has a fine-grained, euhedral or acicular habit, suggesting rapid crystallization from a melt during the metal–silicate mixing. By contrast, the other group (Group 2) is intimately associated with the decomposition and reduction of pyroxene under subsolidus conditions. The Group 1 phosphates yield a precise Pb–Pb age of 4528 ± 11 Ma and they provide the first robust constraint on the timing of impact-induced metal–silicate mixing on the IIE iron meteorite parent body. The Group 2 phosphates, on the other hand, probably record a Pb–Pb age at least 50 Myr younger, and they give insight into the role of subsequent impacts on the alteration of IIE irons. Combining our results with published chronological data suggests that the formation of IIEs irons is consistent with early partial differentiation followed by multiple impact-driven metal–silicate mixing and that most of them were not shielded from subsequent impacts.

Evidence of crustal disequilibrium melting, mingling processes, layering and deformation in UHT conditions: The marginal zone of the Egersund-Ogna massif-type anorthosite (S. Norway)

Septa of banded gneisses crop out along the margin of the Proterozoic Sveconorwegian Egersund-Ogna (EGOG) anorthosite pluton. They permit the observation of the effects of a diapiric intrusion of anorthosite on its envelope of gneisses. The rocks are mylonitic and result from a strong deformation in UHT granulite facies conditions due to contact metamorphism. Concordance of the foliations in the septa and in the leuconoritic margin of the anorthosite suggests that the deformation was linked to the ballooning of the anorthosite. The septa lithology is dominated by noritic gneisses interleaved with felsic gneisses and comprises minor amounts of mafic granofels, metaquartzites and metagreywackes. A charnockitic dyke is cutting across the foliated structure. Geochemical data (major and trace elements) indicate that the noritic gneisses did not result from the metamorphism of amphibolites belonging to the high-grade gneiss complex characteristic of the wall rocks of the Rogaland anorthosite province. Instead they have strong similarities with a high-alumina basalt that is the possible parental magma of the EGOG anorthosite. Slight variations from norite to leuconorite compositions were induced by metamorphic differentiation. The felsic rocks encompass a large interval of rock types from granite to tonalite and quartz diorite. They compare with leucogranitic enclaves occurring in the charnockitic upper part of the Bjerkreim-Sokndal layered intrusion. Most felsic lithologies show the signature (including high positive Eu anomalies) of anatectic processes in disequilibrium melting conditions and rapid extraction from the source rocks. Flowage differentiation in the emplacement process explains the various compositions, as commonly observed in migmatite leucosomes. The metaquartzites show a relatively high Ti concentration and the Ti-in-quartz geothermometer records a minimum temperature of 820 °C. The study brings evidence that the high-alumina basalt and the felsic magmas were aggregated but did not interacted to yield hybrid magmas, possibly because the time interval was too short. They mingled in various proportions to form the norito-felsic gneisses. The aggregated magmas were intimately linked to the EGOG intrusion in which they played a role as a lubricating agent. They were injected into the wall rocks and were deformed by ballooning from superheated UHT conditions down to subsolidus conditions where mylonitic structures developed. Finally, a charnockitic magma was still able to intrude at a lower temperature below the ductile-brittle transition.

Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

Sulfides are the most abundant inclusions in diamonds and a key tool for dating diamond formation via Re-Os isotopic analyses. The manner in which fluids invade the continental lithospheric mantle and the time scale at which they equilibrate with preexisting (protogenetic) sulfides are poorly understood yet essential factors to understanding diamond formation and the validity of isotopic ages. We investigated a suite of sulfide-bearing diamonds from two Canadian cratons to test the robustness of Re-Os in sulfide for dating diamond formation. Single-crystal X-ray diffraction (XRD) allowed determination of the original monosulfide solid-solution (Mss) composition stable in the mantle, indicating subsolidus conditions of encapsulation, and providing crystallographic evidence supporting a protogenetic origin of the inclusions. The results, coupled with a diffusion model, indicate Re-Os isotope equilibration is sufficiently fast in sulfide inclusions with typical grain size, at mantle temperatures, for the system to be reset by the diamond-forming event. This confirms that even if protogenetic, the Re-Os isochrons defined by these minerals likely reflect the ages of diamond formation, and this result highlights the power of this system to date the timing of fluid migration in mantle lithosphere.

Tectono-metamorphic evolution of the proto-Andean margin of Gondwana: Evidence of internal high-grade metamorphism along the northern portion of the Famatinian orogen, Sierra de Aconquija, Sierras Pampeanas Orientales, Argentina

The present work gives a detailed analysis of the metamorphic and structural evolution of the back-arc portion of the Famatinian Orogen exposed in the southern Sierra de Aconquija (Cuesta de La Chilca segment) in the Sierras Pampeanas Orientales (Eastern Pampean Sierras). The Pampeanas Orientales include from north to south the Aconquija, Ambato and Ancasti mountains. They are mainly composed of middle to high grade metasedimentary units and magmatic rocks.At the south end of the Sierra de Aconquija, along an east to west segment extending over nearly 10 km (Cuesta de La Chilca), large volumes of metasedimentary rocks crop out. The eastern metasediments were defined as members of the El Portezuelo Metamorphic–Igneous Complex (EPMIC) or Eastern block and the western ones relate to the Quebrada del Molle Metamorphic Complex (QMMC) or Western block. The two blocks are divided by the La Chilca Shear Zone, which is reactivated as the Río Chañarito fault.The EPMIC, forming the hanging wall, is composed of schists, gneisses and rare amphibolites, calc– silicate schists, marbles and migmatites. The rocks underwent multiple episodes of deformation and a late high strain-rate episode with gradually increasing mylonitization to the west. Metamorphism progrades from a M1 phase to the peak M3, characterized by the reactions: Qtz + Pl + Bt ± Ms → Grt + Bt2 + Pl2 ± Sil ± Kfs, Qtz + Bt + Sil → Crd + Kfs and Qtz + Grt + Sil→ Crd. The M3 assemblage is coeval with the dominant foliation related to a third deformational phase (D3).The QMMC, forming the foot wall, is made up of fine-grained banded quartz – biotite schists with quartz veins and quartz-feldspar-rich pegmatites. To the east, schists are also overprinted by mylonitization. The M3 peak assemblage is quartz + biotite + plagioclase ± garnet ± sillimanite ± muscovite ± ilmenite ± magnetite ± apatite.The studied segment suffered multiphase deformation and metamorphism. Some of these phases can be correlated between both blocks. D1 is locally preserved in scarce outcrops in the EPMIC but is the dominant in the QMMC, where S1 is nearly parallel to S0. In the EPMIC, D2 is represented by the S2 foliation, related to the F2 folding that overprints S1, with dominant strike NNW – SSE and high angles dip to the E. D3 in the EPMIC have F3 folds with axis oblique to S2; the S3 foliation has striking NW – SE dipping steeply to the E or W and develops interference patterns. In the QMMC, S2 (D2) is a discontinuous cleavage oblique to S1 and transposed by S3 (D3), subparallel to S1. Such structures in the QMMC developed at subsolidus conditions and could be correlated to those of the EPMIC, which formed under higher P-T conditions. The penetrative deformation D2 in the EPMIC occurred during a prograde path with syntectonic growth of garnet reaching P-T conditions of 640°C and 0.54 GPa in the EPMIC. This stage was followed by a penetrative deformation D3 with syn-kinematic growth of garnet, cordierite and plagioclase. Peak P-T conditions calculated for M3 are 710°C and 0.60 GPa, preserved in the western part of the EPMIC, west of the unnamed fault.The schists from the QMMC suffered the early low grade M1 metamorphism with minimum PT conditions of ca 400°C and 0.35 GPa, comparable to the fine schists (M1) outcropping to the east. The D2 deformation is associated with the prograde M2 metamorphism. The penetrative D3 stage is related to a medium grade metamorphism M3, with peak conditions at ca 590°C and 0.55 GPa.The superimposed stages of deformation and metamorphism reaching high P-T conditions followed by isothermal decompression, defining a clockwise orogenic P-T path. During the Lower Paleozoic, folds were superimposed and recrystallization as well as partial melting at peak conditions occurred. Similar characteristics were described from the basement from other Famatinian-dominated locations of the Sierra de Aconquija and other ranges of the Sierras Pampeanas Orientales.

Metamorphic evolution and Petrogenesis of garnet–corundum silica–undersaturated metapelitic granulites: A new case study from the Mianlüe Tectonic Zone of South Qinling, Central China

Newly identified garnet–corundum silica-undersaturated pelitic granulites in the Mianlüe Tectonic Zone of the South Qinling belt, central China, preserve a distinct mineral assemblage of garnet + corundum + biotite + plagioclase ± kyanite + sillimanite + staurolite and are therefore useful for deciphering the metamorphic evolution of the zone. Most of the garnet porphyroblasts preserve prominent major- and trace-element zoning, which provides evidence for two stages of garnet growth, under subsolidus conditions and then fluid-absent conditions, during prograde metamorphism. The distinctive yttrium annuli in garnet grains are most likely the result of the breakdown of xenotime. Four stages in the pressure–temperature (P–T) evolution of the granulites are inferred from the results of P–T pseudosection analysis together with petrographic observations. The first stage records the early prograde evolution, comprising isobaric heating at ~7 kbar from 580 to ~700 °C, and the later-stage prograde evolution with increasing pressure and temperature to the peak-pressure stage. The second stage is marked by the maximum XGrs in the garnet mantle, indicating the peak conditions in the high-P–high-T granulite facies of 830 °C at ~11.7 kbar. The third stage is characterized by a continued increase in pyrope in the garnet rim, suggesting decompression with slight heating from the peak pressure to reach the peak temperature at ~9.4 kbar and ~850 °C. The late retrograde cooling stage is represented by further decompression after the peak temperature, accompanied by the growth of staurolite. 7–13 mol% melt was produced during prograde metamorphism via the reaction sill + bi + pl → g + crn + melt. Although later melt loss reduced the bulk-rock Si and Al, the low-Si and high-Al–Fe bulk compositions of restite were inherited mainly from the protolith, which comprised high-Fe sediments (bauxites) derived from weathering of adjacent mafic rock. U–Pb dating of metamorphic zircon rims yielded two age clusters: ~227 Ma represents the timing close to peak-pressure conditions, and 222–217 Ma represents the timing of post-Tmax cooling to the solidus. Monazite U–Pb ages of 212–206 Ma are interpreted as the timing of further retrograde cooling under the solidus. The geochronological data and clockwise P–T paths indicate the garnet–corundum pelitic granulites record a complete metamorphic P–T–t history, including crustal thickening during continental collision/subduction and subsequent exhumation and cooling during the early Mesozoic. Combined with recent high-precision early Neoproterozoic data, we proposed that the MTZ was formed as a result of Grenvillian oceanic plate subduction and the assembly of Rodinia during the early Neoproterozoic and that it also records processes related to continental collision/subduction during the early Mesozoic.

Micropores and mass transfer in the formation of myrmekites

Abstract The formation process of myrmekites in granitic rocks can help us to understand the mass transfer between minerals and hydrothermal fluids during the deuteric stage. The Toki granite, central Japan, has three types of myrmekites. Type A myrmekite is defined as a single layer. Type B myrmekite shows a composite texture consisting of two layers, namely, a myrmekite layer and an albite (Ab)-rich layer that is free of vermicular quartz. Type C has a composite texture with the following three layers: two myrmekite layers separated by one Ab-rich layer. Micropores are found in these myrmekites in the undeformed granite, which enable quantitative determinations of the volume decrease during myrmekite formation by measurement of the area of micropores. The areal relationship between the micropores and vermicular quartz in the myrmekites exhibited a high correlation (R2 = 0.8352), thus indicating that the genesis of the micropores is evidently related to myrmekite formation. We derived the reaction equations for myrmekite formation based on the singular value decomposition method. The matrices for singular value decomposition involve the following volume factors: volume change during the reaction and volume ratios of the product minerals. The singular value decomposition indicates that the myrmekites are produced through the consumption of plagioclase and K-feldspar with an inflow of H4SiO4, Na+, and H+ from the hydrothermal fluid, accompanied by an outflow of Al3+, Ca2+, and K+ into the fluid, which constitute essential mass transfers during myrmekite formation. The difference of pH in the hydrothermal fluid and inflow amounts of H4SiO4 and Na+ can explain the reason why micropores occur in the myrmekites but not in the Ab-rich rim (layer); the smaller inflow of H4SiO4 and Na+ from the hydrothermal fluid with lower pH conditions yielded micropore production during myrmekitization, and the larger inflow of H4SiO4 and Na+ from the hydrothermal fluid with higher pH conditions yielded the formation of the Ab-rich rim (layer) with few micropores. The sequential variations in the chemical characteristics of the hydrothermal fluid during the sub-solidus conditions were characterized by a gradual decrease in H4SiO4 (Si4+), Fe2+, Mn2+, Mg2+, and Na+ and a gradual increase in Ca2+, K+, H+, and F–.

Cl-rich amphiboles and micas in rocks of the Middle Riphean Kusa-Kopan complex of mafic layered intrusions (southern Urals, Russia)

High-Cl amphiboles and micas were found in rocks of the Kusa-Kopan complex of the Middle Riphean layered ore-bearing (Ti, Fe, V) mafic intrusions in the Southern Urals, Russia. Chloro-hastingsite (3.30–4.57 wt% Cl) was found in garnet-bearing orthoamphibolite in the Medvedevsky massif. Compositions of Cl-rich (3.26–4.84 wt% Cl) amphibole from borehole # 2 of the Kusinsky massif located to the north, correspond to “potassic-ferro-chloro-pargasite”, in some cases to potassic-chloro-hastingsite. Thus, in addition to the high-K trend in the evolution of Cl-rich amphiboles, a tendency to low-K content was revealed. Apart from amphibole, biotite with Cl content from 1.73 to 2.32 wt% corresponding to the annite-phlogopite series was observed. Calculations showed that Cl content reached 1.4 wt% in the melt/fluid during the crystallization of the high-Cl amphiboles of the Kusa part of the layered intrusion, and was about 6000 ppm during the crystallization of the “chloro-hastingsite” from the Medvedevsky massif. The same effect could be seen in lower-Cl varieties, where the differences reached a factor of three. The Cl content in the initial melt did not exceed 400 ppm, locally increasing up to 1000–4000 ppm. Chlorine-rich amphiboles are the result of a local excess concentration of chlorinated compounds of complex composition at the final stage of the magmatic system development (under subsolidus conditions). The emergence of an open system cannot be ruled out, in which Cl content in the fluid increases over time resulting from an external source, which is enriched in Cl due to the addition of mobilized brines from the host rocks additionally enriched with CO2.

Experimental Study of Pt Solubility in the CO-CO2 Fluid at Low fO2 and Subsolidus Conditions of the Ultramafic-Mafic Intrusions

The solubility of Pt in CO-CO2 fluid was studied experimentally at P = 50–200 MPa and T = 950 °C. A mixture of MgC2O4 and MgCO3 was used as a source of the fluid. Upon the reaction of the Pt capsule walls and the fluid, a carbonyl of platinum is formed. The use of the high-temperature quartz ceramics as a fluid trap avoids the effect of mechanical contamination with Pt from the eroded capsule walls. The total content of platinum in the porous fluid traps was measured by the Electrothermal Atomic Absorption (ET-AAS) method. In some experiments, the local analysis of traps was carried out by the Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) method. The composition of fluid in bubbles captured in an albite glass trap was studied by micro-Raman spectroscopy. On the capsule walls and MgO, Pt “whiskers” of submicron diameter were observed, which were formed as a product of carbonyl decomposition during quenching. About 5–15% of carbonyl withstands quenching resulting in 1.5 to 2 ppm Pt soluble in acetone (runs at P = 200 MPa) in a quartz glass trap. The amount of Pt soluble in acetone from the capsule walls corresponds to a concentration of up to 8 ppm in the fluid. A high content of soluble Pt of 2000–3000 ppm was determined in a carbon coated MgO matrix. Our study demonstrated that the solubility of Pt in the CO-CO2 fluid is 15–150 ppm, presumably in the form of Pt3(CO)62− under conditions corresponding to the conditions of the subsolidus stage of layered ultramafic-mafic and ultramafic-alkaline intrusions formation. Our preliminary data showed that this solubility will increase with the addition of water at low fO2.

Deep Tectonics in the Eastern Hellenides Uncovered: The Record of Variscan Continental Amalgamation, Permo‐Triassic Rifting, and Early Alpine Collision in Pre‐Variscan Continental Crust in the W‐Rhodope (Vertiscos‐Ograzden Complex, N‐Greece)

AbstractThe Vertiskos‐Ograzden metamorphic complex (VOC; W‐Rhodope Metamorphic Domain, RMD) is a key area for reconstructing the geodynamic history of the Hellenides. We distinguish four metamorphic events: (1) an (U)HP eclogite facies metamorphism, succeeded by partial melting during decompression, amphibolite facies reequilibration in the kyanite stability field, and injection of muscovite pegmatites; (2) a LP‐HT event, associated with partial melting within the stability field of sillimanite, cordierite, and K‐feldspar (andalusite at subsolidus conditions) and magmatic intrusions; (3) a prograde moderate high‐pressure (MHP) metamorphism within the kyanite stability field increasing to the NE from epidote amphibolite to upper amphibolite facies; and (4) in the easternmost VOC, a LP amphibolite facies event (new cordierite and andalusite). Considering existing geochronological data, the VOC formed part of successive tectonic settings: (1) the “Variscan” collision zone between “East‐Avalonian” and Armorican continental fragments; (2) the Triassic middle crust of a continental rift zone, (leading to formation of oceanic crust, in the eastern VOC); (3) late Jurassic‐early Cretaceous closure of the oceanic basin and accretion of the VOC to southern European margin and northeastward polarity of subduction/collision; (4) late Cretaceous extension and crustal thinning. Then, subduction of the Neo‐Tethys and Apulian plate underneath Europe occurred, and the VOC plus the upper units of the RMD were part of the upper (European) plate. Large‐scale north dipping low‐angle normal detachments exhumed the VOC from the vicinity of a magmatic arc presently situated in the north RMD and Sredna Gora and Strandja zones and drag it into its present position.

Shear deformation of nano- and micro-crystalline olivine at seismic slip rates

Several thermally activated mechanisms have been proposed to explain the dynamic weakening of faults at fast, seismic slip rates v > 0.1 m s−1. Nevertheless, direct constraints from microstructural observations on the operation of such weakening mechanisms are still lacking for most rock forming minerals. This includes olivine, a very refractory mineral relevant for seismic activity occurring in peridotite massifs. Here, we report mechanical data and microstructures of olivine aggregates deformed in a rotary shear apparatus at slip rates from 10−2 m s−1 to 1 m s−1. A number of 34 shear deformation experiments were performed under axial stresses of 20 MPa and at room temperature. Samples are composed of either synthetic iron-free nanocrystalline forsterite powder (initial grain size = 0.07 μm) or natural iron-bearing olivine powder from San Carlos or San Bernardino (Fo91, Arizona, USA; initial grain size = 70 ± 2 μm). Shear deformation lasts from a few seconds up to 5 min and ends after 0.03 to 3 m of slip. Independently of the initial grain size or composition, friction coefficients decrease from peak values of 0.8 to values below 0.4, after only 0.1 m of slip. The recovered samples were characterized by scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. For all samples, before the onset of dynamic weakening, the deformation localizes in a principal shear zone, well-marked in the natural olivine aggregates by a rapid decrease of grain size down to <1.7 μm. Variations in texture, fractured grains and the formation of a fine-grained matrix support cataclastic flow at an early stage of deformation, responsible for the foremost grain size reduction. The samples display well-defined zones with partially sintered microstructure, and shape preferred orientation. The initial texture of the cold-pressed starting material is rapidly reduced to a near random distribution after only 0.03 m of slip. All crystal preferred orientations (CPO) remain weak, with a J-index ≲2. Based on the microstructures, the dominant deformation mechanism may be enhanced effective diffusion (by surface and grain boundary diffusion) causing mechanical weakening and a viscous behavior, which is then temperature-dependent under sub-solidus conditions. We find no microstructural evidence for frictional melting, flash heating or thermal decomposition. Our results advocate that dynamic weakening of faults is independent of initial grain size, and can be achieved after displacements as low as 0.1 m without production of frictionally induced-melt.

The effects of solid-solid phase equilibria on the oxygen fugacity of the upper mantle

AbstractDecades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle.Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇄ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations.Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

Submagmatic to Solid-State Deformation Microstructures Recorded in Cooling Granitoids during Exhumation of Late-Variscan Crust in North-Eastern Sicily

Late-Variscan granitoid rocks of trondhjemitic and granitic composition, intruded in migmatitic paragneisses in the north-eastern Peloritani Mountains (southern Italy) at ~310 Ma and ~300 Ma, respectively, exhibit a range of deformation microstructures developed under a shear regime at decreasing temperatures. Non-coaxial deformation is documented by sigmoidal feldspar porphyroclasts, mica fish, and asymmetric boudins affecting tiny andalusite crystals. Late-Variscan shearing during granitoid cooling is constrained by largely represented chessboard patterns in quartz and, especially, submagmatic fractures in plagioclase, indicating deformation at high-temperature conditions (T &gt; 650 °C), in the presence of melt. Submagmatic deformation was extensively superseded by deformation at lower temperatures. Examples of solid state-high temperature deformation-related microstructures (T &gt; 450 °C) include feldspar bulging, quartz grain boundary migration, and subgrain rotation recrystallization. Finally, low temperature subsolidus microstructures (T &lt; 450 °C) consist of quartz bulging, mica kinks, and feldspar twinning and bending. A complete sequence of deformation, operating from submagmatic to low-temperature subsolidus conditions is recorded in both the older and younger granitoids, suggesting a duration of ~20 Ma for shear zone activity during post-collisional exhumation of the Variscan middle crust in southernmost Italy.

Melt inclusions at MT. Edixon (Antarctica): Chemistry, petrology and implications for the evolution of the Lanterman range

The recovery of melt inclusions (MI) in peritectic phases of metamorphic rocks demonstrates their anatectic origin even in the absence of other textural or field evidences, and gives the opportunity to retrieve the chemical composition of primary melts and infer melt entrapment conditions. Here we report the first recovery of such inclusions in a medium-grade schist from the Edixon Metamorphic Complex (EMC) of the Lanterman Range in northern Victoria Land, Antarctica. This is the first study of melt inclusions from an Al2SiO5-free, graywacke-type protolith. Inclusions are hosted in garnet. They are both polycrystalline nanogranitoids, i.e., crystallized former MI, and fluid inclusions (FI) coexisting in the same clusters and displaying frequent negative crystal shape and textures indicative of a primary origin. The MI contain albite, K-feldspar, quartz, muscovite, biotite ± calcite ± siderite ± H2O, while the FI are composed by CO2 + H2O + N2 + CH4 ± H2S and contain step-daughter calcite.The schist hosting the garnets with melt inclusions contains muscovite, biotite, garnet, plagioclase, K-feldspar, and quartz. Extensive resorption of garnet and replacement by biotite, along with composition of micas that suggest medium-temperature, subsolidus conditions, indicate that after anatexis the schist was thoroughly recrystallized under retrograde conditions. The MI were remelted in a piston cylinder apparatus at conditions of 740–900 °C at 0.8–1.0 GPa with complete remelting taking place at 760–780 °C. The remelted glass is sub-alkaline, peraluminous (ASI ≈ 1.3–1.5) with CaO ≈ 1.2–2.1 wt% and K2O ≈ 3.4–4.1 wt% and contains ≈ 4.3–5.3 wt% of H2O and 400–1150 ppm of CO2. The P-T conditions of melt entrapment were retrieved combining results of experimental remelting with calculated phase equilibria in the MnCNKFMASHT model system, and are between 740 and 780 °C and 0.7 to 0.9 GPa. The new and different P-T conditions for a high grade rock of the EMC with respect to other published data, confirm that this metamorphic complex is not homogeneous, and that anatexis may have taken place at different crustal levels during the evolution of the Cambro-Ordovician Ross Orogen.

Classification and characterisation of magmatic-hydrothermal tourmaline by combining field observations and microanalytical techniques

Tourmaline from the St. Byron lobe of the Land’s End granite, SW England, was assessed by macroscopic, optical and quantitative microanalytical methods. In total, seven types of tourmaline were distinguished. The seven types reflect different crystallisation environments and stages in the magmatic-hydrothermal transition. Types 1-3 are interpreted to represent a gradual transition from tourmaline crystallising from a silicate melt to precipitation from magmatic aqueous fluids. Types 5-7 crystallised at subsolidus conditions from a different fluid generation than types 1-3. These fluids may be magmatic or mixed with other fluids (e.g., meteoric or formation waters). The Sn-mineralisation in the area is mostly related to the latter fluid generation, and the mineralising potential is reflected by the tourmaline composition.

Metamorphic Response to Alpine Thrusting of a Crustal-scale Basement Nappe in Southern Calabria (Italy)

Abstract Structural, geophysical and age data indicate that the tilted cross-section of the Variscan continental crust exposed in the Serre of southern Calabria forms the uppermost Alpine nappe (‘Serre nappe’) of three Variscan basement slices derived from the southern European margin. This Alpine nappe stack is a fragment of the western Mediterranean Alps and rests now, after Miocene emplacement, on top of the Apennine carbonate platform. We report for the first time a P–T path for prograde Alpine metamorphism, which is restricted to the two lower nappes (Castagna and Bagni nappes) that are squeezed between cooler tectonic units, the Serre nappe above and the Apennine platform below. Therefore, we attribute their metamorphism to tectonic loading and concomitant shear heating during Eocene south-directed overthrusting of the crustal-scale Serre nappe. In the underlying Castagna nappe, Alpine metamorphism is only locally recorded, mainly by new growth of garnet, forming at the expense of retrogressed Variscan biotite dated at 43 Ma. The local existence of Alpine besides relict Variscan mineral assemblages in the strongly but heterogeneously overprinted rocks allows for characterization of metamorphic evolutions during both the Alpine and Variscan orogeneses in the former intermediate level of the Variscan crust of Calabria. The metamorphic evolutions have been reconstructed through P–T pseudosection modelling for Al-rich metasediments. In the Castagna nappe, rarely preserved Variscan garnet–sillimanite–biotite–ilmenite–plagioclase–quartz (± K-feldspar ± Si-poor white mica) assemblages formed under amphibolite-facies subsolidus conditions (650 ± 60 °C, 4·0 ± 0·5 kbar). During subsequent decompression and cooling to greenschist-facies conditions garnet was replaced by biotite–sillimanite and later by white mica–chlorite intergrowths. Retrogression of Variscan biotite is evidenced by the exsolution of ilmenite along grain boundaries and cleavages, textures that were subsequently overgrown by Alpine garnet coexisting with Si-rich white mica, rare chloritoid (in metapelites), and hornblende (in metagreywackes). Alpine garnet shows prograde zoning and is Ca-rich, and thus is distinct from unzoned and Ca-poor Variscan garnet porphyroblasts. Estimated conditions (520 ± 40 °C, 8·0 ± 1·0 kbar) record elevated pressures during Alpine lower amphibolite-facies metamorphism. In the lowermost Bagni nappe, rare prograde-zoned, Ca-rich garnet in strongly retrogressed mylonitic quartz-phyllites allows isopleth thermobarometry, which indicates lower amphibolite-facies conditions (555 ± 10 °C, 7·4 ± 0·3 kbar) resembling those for Alpine garnet growth in the Castagna nappe. The similar clockwise P–T paths for prograde Alpine metamorphism and the consistent peak pressures of 7–9 kbar in the Castagna and Bagni nappes point to a joint short-lived metamorphism during overthrusting of the crustal-scale Serre nappe within the south European margin during the north-directed subduction of the Alpine Tethys. South-directed overthrusting of the now tilted Variscan crustal section of the Serre along the up to 500 m thick mylonite horizon of the Curinga–Girifalco Line is in agreement with seismic data indicating an extended, few kilometres thick low-velocity zone (Bagni and Castagna nappes and mylonites of the Curinga–Girifalco Line) below the exposed lower crustal section of the Serre nappe. Alpine tectonic transport direction, timing and metamorphic conditions described here are consistent with those reported from the Aspromonte area in southernmost Calabria, suggesting a coeval Alpine history characterized by metamorphism owing to nappe loading and concomitant shear heating. The Alpine subduction–erosion–accretion processes inferred here for the Calabrian basement nappes resemble those proposed for the Dent Blanche nappe system in the Western Alps.

Lithological and structural controls on the emplacement of a Neoarchean plutonic complex in the Carajás province, southeastern Amazonian craton (Brazil)

New geological and geochronological (Pb-Pb evaporation) data from the Vila Jussara suite, a representative of the Neoarchean granitoids of the Carajás province, southeastern Amazonian craton (Brazil), are discussed. The Vila Jussara suite crops out as a series of coalescing plutons elongated in the E-W direction. These plutons generally have sigmoidal shapes, and steep dips (75–85°) that follow the regional trend. The centermost areas of plutons are generally slightly deformed, while the marginal portions display mylonitic aspect and are delimited by sinistral shear zones belonging to the transcurrent system of the Itacaiúnas shear belt. These granitoids cover a large compositional spectrum, with four main lithotypes: (i) Seriated biotite-hornblende monzogranite; (ii) biotite-hornblende tonalite; (iii) porphyritic biotite monzogranite-granodiorite; and (iv) porphyritic hornblende-biotite monzogranite-granodiorite. The microstructural analysis of the studied rocks reveals: (i) microfractures in feldspar phenocrysts, filled with a quartz-feldspar matrix; (ii) preferred orientation of euhedral feldspar crystals in an igneous matrix; and (iii) evidence of moderate-to high-temperature of solid-state deformation (>500 °C). These microstructures and field observations point to a continuum of deformation, from the (sub) magmatic state down to sub-solidus conditions. Dating of three types of granitoids out of four gives ages of 2754 ± 2.2 Ma, 2752 ± 5.7 Ma and 2745 ± 3 Ma. The different granitoid varieties of the Vila Jussara suite were thus emplaced simultaneously. It is proposed that emplacement of the granitoids was controlled by shear zones reactivated during oblique collision and that plutons where constructed by multiple injections of magmas, generating extensive hybridization.

Nepheline syenite intrusions from the Rengali Province, eastern India: Integrating geological setting, microstructures, and geochronological observations on their syntectonic emplacement

Ten nepheline syenite intrusions have been concordantly emplaced within or close to a polydeformed, regionally metamorphosed sedimentary sequence in the Rengali Province, eastern India during the Mesoproterozoic. The intrusions and the country rocks share a common deformation history and record three phases of folding. The nepheline syenites preserve deformation structures and textures indicative of continuous deformation that started in the presence of a melt and ended under sub-solidus conditions. This deformation in nepheline syenites is divided on the basis of mechanical behavior into three stages: an early one with the development of varied types of magmatic layering and flattened magmatic enclaves, a middle one with the development of asymmetrical magmatic folds and intramagmatic shear zones, and a late stage with melt-enhanced embrittlement. Igneous zircons from two of the intrusions, namely Chhatabar and Gunjighara are dated at 1356 ± 5 Ma and 1358 ± 4 Ma (U–Pb concordant age, LA–ICP–MS), respectively. Electron microprobe U–Th–total Pb geochronology of monazites from two samples of sillimanite grade mica schists in the host metasedimentary sequence of the Chhatabar nepheline syenite yields metamorphic ages of 1362 ± 12 Ma and 1376 ± 23 Ma, which indicate broad synchronism of regional metamorphism of the country rocks and the nepheline syenite magmatism. Contrary to the common proposition that nepheline syenite intrusions in eastern India and in some other regions of Precambrian crust were emplaced anorogenically in intracontinental rifts and subsequently deformed by continental collision, we develop a structural interpretation in which the nepheline syenites were emplaced syntectonically in the sense that magma transport and cooling occurred during regional metamorphism and deformation. The study shows that an integration of structural, textural and geochronological data from both nepheline syenite and country rock is crucial for an understanding of the spatial and temporal relationships between contractional deformation and alkaline magmatism. Our results imply that deformed nepheline syenites may not always be useful in constraining the initiation of a Wilson Cycle heralded by intracontinental rifting.

Magmatic and Metasomatic Effects of Magma–Carbonate Interaction Recorded in Calc-silicate Xenoliths from Merapi Volcano (Indonesia)

Abstract Magma–carbonate interaction is an increasingly recognized process occurring at active volcanoes worldwide, with implications for the magmatic evolution of the host volcanic systems, their eruptive behaviour, volcanic CO2 budgets, and economic mineralization. Abundant calc-silicate skarn xenoliths are found at Merapi volcano, Indonesia. We identify two distinct xenolith types: magmatic skarn xenoliths, which contain evidence of formation within the magma; and exoskarn xenoliths, which more likely represent fragments of crystalline metamorphosed wall rocks. The magmatic skarn xenoliths comprise distinct compositional and mineralogical zones with abundant Ca-enriched glass (up to 10 wt % relative to lava groundmass), mineralogically dominated by clinopyroxene (En15-43Fs14-36Wo41-51) + plagioclase (An37-100) ± magnetite in the outer zones towards the lava contact, and by wollastonite ± clinopyroxene (En17-38Fs8-34Wo49-59) ± plagioclase (An46-100) ± garnet (Grs0-65Adr24-75Sch0-76) ± quartz in the xenolith cores. These zones are controlled by Ca transfer from the limestone protolith to the magma and by the transfer of magma-derived elements in the opposite direction. In contrast, the exoskarn xenoliths are unzoned and essentially glass-free, representing equilibration at sub-solidus conditions. The major mineral assemblage in the exoskarn xenoliths is wollastonite + garnet (Grs73-97Adr3-24) + Ca-Al-rich clinopyroxene (CaTs0-38) + anorthite ± quartz, with variable amounts of either quartz or melilite (Geh42-91) + spinel. Thermobarometric calculations, fluid-inclusion microthermometry and newly calibrated oxybarometry based on Fe3+/ΣFe in clinopyroxene indicate magmatic skarn xenolith formation conditions of ∼850 ± 45°C, &amp;lt; 100 MPa and at an oxygen fugacity between the NNO (nickel–nickel oxide) and HM (hematite-magnetite) buffer. The exoskarn xenoliths, in turn, formed at 510–910°C under oxygen-fugacity conditions between NNO and air. These high oxygen fugacities are likely imposed by the large volumes of CO2 liberated from the carbonate. Halogen- and sulphur-rich mineral phases in the xenoliths testify to infiltration by a magmatic brine. In some xenoliths, this is associated with the precipitation of copper-bearing mineral phases by sulphur dissociation into sulphide and sulphate, indicating potential mineralization in the skarn system below Merapi. The compositions of many xenolith clinopyroxene and plagioclase crystals overlap with that of magmatic minerals, suggesting that the crystal cargo in Merapi magmas may contain a larger proportion of skarn-derived xenocrysts than previously recognized. Assessment of xenolith formation timescales demonstrates that magma–carbonate interaction and associated CO2 release could affect eruption intensity, as recently suggested for Merapi and similar carbonate-hosted volcanoes elsewhere.